A variety of orthopedic devices for controlled relative adjustment of the separation and orientation of bone segments are known in the art. For example, distraction devices (commonly referred to as distractors) are known. Typically, distractors are used to perform distraction osteogenesis. This procedure was perfected by the Russian orthopedic doctor, Gavriel Ilizarov. A typical procedure of this type involves at most an osteotomy completely separating the bone into two segments, or at least an incision of the cortical portion of the bone. Then, the bone segments on either side of the bone repair site may be incrementally separated. As used herein "bone repair site" refers to any bone region which is desired to be treated using the orthopedic system and which is bounded on opposing sides by relatively healthy bone regions to which orthopedic devices can be secured. The bone repair site will often be an osteotomy or a fracture. The incremental separation of the bone segments allows new bone to form in the osteotomy void; this is variously referred to as osteosynthesis or osteogenesis, among other terms. This incremental "distraction phase" is followed by a "consolidation phase," during which the distractor is held fixed, and the new bone growth gains strength. Following the consolidation phase, the distractor is removed from the patient. In addition to distractors, it is also known to provide reduction devices, or "reducers," for controllably and gradually bringing separated bone segments together.
The design of the early reducers and distractors, which used bone pins and screws to attach the externally-mounted devices to the bone, are known to have certain problems. For example, numerous pins are needed to attach a single device; at a minimum, two pins are required (one for the part of the device associated with each bone segment), but typically, many more are used. Each pin necessitates a transcutaneous incision, thus multiplying the risk of infection to the patient. Furthermore, a pin clamp or coupling is required to join the fixation/distraction mechanism to the pins, and the design and operation of these couplings are complicated by the difficulty encountered in aligning the pins accurately when they are inserted into the bone. Additionally, because the pins extend in a generally perpendicular direction from the insertion site, the resultant overall device sticks out quite far from the patient's body (i.e., the device has a high profile), and the device is unsightly. A high-profile device also is more subject to bumps and snags than one maintained tight to the patient's body. Further, aesthetic considerations are important because a high-profile device may be rejected by prospective patients, especially children.
Accordingly, more recent devices have been designed not to use transcutaneous pins for attachment to the bone, but rather to use more low-profile bone anchors, such as plates with screw holes, as part of a low-profile overall device. This improved prior art is exemplified by U.S. Pat. No. 5,364,396 to Robinson et al., which discloses an implantable bone distraction device which includes low profile blocks for attachment to osteotomically separated bone sections. As disclosed, the entire device can be implanted subcutaneously, except for a transcutaneous actuator assembly which is linked to the implanted distraction assembly, and allows adjustment of the distraction increment from outside the patient's body. The difficulty with this device is that it requires the subcutaneous implantation of a rather large mechanism, with the result that it is difficult to hide the presence of the implant.
U.S. Pat. No. 3,604,414 to Borges also discloses an implantable bone reduction device designed to be adjusted using an external apparatus. The reduction plate is of two-piece construction, with the plates incorporating opposing toothed sections. The toothed sections prevent relative movement of the plates in the direction of fracture separation, but allow movement in the direction of fracture reduction. A tool for moving the plates toward each other is provided to facilitate engagement of the plates after they have been attached to the opposing segments of bone. The difficulty with this device is that it requires the osteotomy site to be fully uncovered each time an adjustment is made, since the adjustment tool resembles a caliper, and engages the opposing plates from above, and in a perpendicular orientation, which requires full access to the face of each plate. Such an invasive surgery increases the likelihood for infection and scarring.
U.S. Pat. No. 5,672,167 to Seldin and U.S. Pat. No. 5,827,286 to Incavo et. al. disclose implantable bone distraction devices designed to be adjusted without first reopening the osteotomy site. Both disclose devices comprising two plates, each of which is attached to a bone segment, and which plates engage one another. The devices are designed to operate in a ratcheting manner in response to the extracorporeal application of force. These ratchets allow relative movement of the plates in one direction along the system axis, but not the reverse. The Seldin device is actuated by the application of force to a pre-tensioned ratchet arm incorporated into the first plate. This ratchet arm contacts a corresponding ratchet pawl on the second plate and forces the two plates apart, thereby achieving distraction of the attached bone segments. The ratchets then prevent the plates from assuming their previous position. The difficulty with the Seldin device is that it provides an imprecise and inaccurate means of measuring bone distraction. The Seldin device has a further disadvantage of requiring that the external actuating force be applied perpendicular to the bone axis. This perpendicular force may tend to cause increased patient discomfort. Additionally, the Seldin device requires the actuating force be applied at or near the suture location, which also may result in increased discomfort, suture healing problems, or scarring. With the Incavo device, bone lengthening is achieved through the use of an external extension brace which is manipulated to apply force directly to the patient's limb. The Incavo device extends in response to this manual distraction, and its ratchet arrangement prevents it and the respective bone segments from returning to their prior position. The difficulty with the Incavo device is that it requires the use of large external alignment apparatus, and may not be adaptable to small bone distractions such as in maxillofacial applications.
Accordingly, there is a need in the art to provide a low profile, aesthetically pleasing, subcutaneous distractor that minimizes the total volume of implant installed, while providing maximum flexibility, precision, and accuracy in adjusting incrementally the distance between separated bone halves.